Over the last few years the technical literature has widely reported so-called cold cages or cold crucibles used in physicochemical applications or special metallurgy, particularly for the melting by high or medium frequency induction of metals or special alloys, or refractory or non-refractory insulating materials, often taking place at elevated temperatures. Most of the cold crucibles described have a number of either straight or hairpin-like segments. Generally the segments are made from copper, which is completely suitable if the melting operation is carried out without direct contact with the crucible under a protective ambient, or if the copper of the crucible is protected from the liquid bath by a protective coating of solid slag or a natural shell of liquid material (insulating under normal conditions) melted by induction at an appropriate frequency and whereof a thin film is solidified on contact with the cold crucible.
This state of the art is represented in FIG. 1, which shows in a perspective elevation a cold crucible, whose cold cage 1 comprises a certain number of copper segments such as 2. This cage 1 is cooled by a circulation of water, diagrammatically represented by inlets 3 and outlets 4 linking the interior of each segment 2 with the exterior. FIG. 1 also shows the induction coil 5, which is responsible for melting the material 6 by high frequency magnetic induction. This material enclosed in the crucible 1 is made internally tight by a solidified crust 7, thus forms to a certain extent the secondary of a transformer, whose inductor 5 is the primary. Under the influence of high frequency secondary currents which occur in material 6, the latter is heated and the sought melting takes place.
However, in a certain number of operations, which is constantly increasing as a result of the great increase in the potential applications of resulting or metallurgical processes in cold crucibles, it may occur when the crucible is immersed in a physically or chemically aggressive environment that the copper is deteriorated either by chemical etching or by the entrainment of copper atoms or particles as a result of surface desorption or erosion. These effects are very prejudicial to the extent that they are liable to bring about premature wear of the crucible or can lead to a pollution of the material treated in the crucible when the latter is used for producing very high purity materials.
These prejudicial phenomena linked with the presence of copper as the base metal of the cold cage are made worse when the latter is used as an enclosure for the confinement or maintaining of an inductive plasma, such applications becoming increasingly numerous. Such inductive plasma torches are ever more widely used for the preparation of metals, alloys or ultra-pure precious materials, such as silica, quartz, alumina, silicon, titanium, etc.